Conventionally, it is known that addition of nitrogen is effective for enhancing corrosion resistance of a martensitic stainless steel, and in order to add a large amount of nitrogen to a martensitic stainless steel, there has been employed a method of pressurizing and dissolving high nitrogen by using a pressure induction furnace, etc. or holding the steel in a high-temperature nitrogen gas atmosphere for a long time, thereby allowing nitrogen to be absorbed from the surface to the inside of a material.
For this purpose, introduction of an expensive melting furnace or a manufacturing process for impregnation with nitrogen is needed.
In order to achieve nitrogen permeation from the surface to the inside of a finished component, the component needs to stored in a high-temperature nitrogen atmosphere for a long time, and this method is mainly targeted at a thin plate-like material and has a problem, for example, that application to a thick component, etc. is difficult.
Further, in the case of a martensitic stainless steel used for a mechanical component such as rolling bearing, it is known that refining of the microstructure is performed so as to obtain high hardness, good rust prevention effect and excellent acoustic properties (quietness).
Patent Document 1 discloses that when C is contained in an amount of more than 0.6% so as to increase the hardness of a high Cr stainless steel, eutectic carbide larger than 10 μm is formed due to large amounts of Cr and C, and the hardness and corrosion resistance are thereby reduced. However, the properties can be improved by appropriately blending alloy elements to refine the eutectic carbide and suppressing the production of δ ferrite.
Patent Document 2 discloses that the strength and toughness can be enhanced by incorporating from 0.1 to 0.2% of W, despite decreasing C compared with SUS440C to a range of 0.35 to 0.45%, and the surface hardness after heat treatment can be made HRc 60 or more by specifying the content of C+N to satisfy 0.60≤(C+N)≤0.65. Further, it is stated that quenchability can be improved by the addition of B and high strength can be obtained by the precipitation of BN.
Patent Document 3 discloses that refinement of the crystal grain size of a martensitic stainless steel to about 30 μm or less can be realized by producing carbonitride to perform pinning. It is stated that a martensitic stainless steel having high surface hardness, high corrosion resistance and high toughness can be obtained by limiting the amounts of precipitates and the martensitic stainless steel can be assured of a Vickers hardness of 550 HV or more.